There is known a flat non-aqueous electrolyte secondary battery such as lithium-ion secondary battery that utilizes a laminate film of a synthetic resin layer and a metal layer as an exterior member so as to accommodate, together with an electrolyte containing a lithium salt etc. (more specifically, a non-aqueous electrolyte having dissolved therein a lithium salt etc.), a battery element in which a plurality of positive and negative electrode plates are formed using lithium composite oxide and carbon material (e.g. graphite) etc. as active materials, respectively, and stacked together via separators.
For the manufacturing of the battery, the battery is subjected to charging operation such as initial charging. During the charging, a strong reducing power (i.e. reducing power that can cause reduction and decomposition of the electrolyte) is generated at the negative electrode plate. The electrolyte is reduced and decomposed under the action of such a reducing power; thereby forming and adhering a coating film onto an active surface of the active material of the negative electrode plate (hereinafter referred to as “negative electrode active material”) (such that the active surface of the negative electrode active material is covered by the coating film). The reduction and decomposition of the electrolyte can be suppressed by the formation of the coating film. Namely, the coating film performs the function of improving various battery characteristics by suppressing the reduction and decomposition of the electrolyte.
Researches have thus been made to form a coating film with better quality, rather than to simply form a coating film by reduction and decomposition of the electrolyte (hereinafter referred to as “electrolyte-derived coating film”), for the purpose of improvements of various battery characteristics. For instance, there is known a technique to add an electrolyte additive (such as vinylene carbonate) to the electrolyte in advance of the charging operation of the battery such that the additive is reduced and decomposed to form a desired coating film (hereinafter referred to as “additive-derived coating film) on the active surface of the negative electrode active material (see e.g. Patent Document 1).